1. Field of the Invention
The present invention relates to a time-of-flight (TOF) mass spectrometer and, more particularly, to a TOF mass spectrometer having an ion detector that is prevented from saturating.
2. Description of the Related Art
An orthogonal acceleration time-of-flight mass spectrometer (OA/TOF-MS) is an instrument for performing a mass analysis by producing ions continuously by an ion source, introducing the ion beam emitted from the ion source into an ion reservoir, accelerating the ions in the ion reservoir in a pulsed manner in a direction orthogonal to the direction of introduction of the ions, and measuring the flight time from the instant when the ions are accelerated to the instants when the accelerated ion pulses are detected by a final ion detector.
FIG. 1 schematically shows the configuration of an OA/TOF-MS employing an electrostatic reflecting mirror. Now let us consider the instrument as shown in FIG. 1 that is polarity positive. It has an external ion source 1 for producing positive ions continuously by electron impact (EI), chemical ionization (CI), fast atom bombardment (FAB), electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), inductively coupled plasma-mass spectrometry (ICP-MS), or other ionization techniques.
An ion beam emitted from the external ion source 1 at a positive accelerating potential V1 is focused in the z direction by a focusing lens 2 to which a positive potential VF is applied. Then, the beam is admitted into an ion reservoir 3 having an effective length y0. The ion reservoir 3 is equipped with a push-out plate 4. The reservoir 3 is also provided with an ion extraction grid 5 and an exit grid 6 that are located opposite to the push-out plate 4. The extraction grid 5 is at ground potential. The exit grid 6 is at a negative potential. Thus, an electric field is developed to push out ions in a direction (z direction) orthogonal to the direction of introduction of the ion beam (y direction).
If a push-out pulse 7 consisting of a positive voltage of 2Vs is applied to the push-out plate 4, a field gradient is momentarily produced in a region 8 that extends from the push-out plate 4 to the exit grid 6 across the ion extraction grid 5. This region 8 is known as the two-stage accelerating region. As a result, the ions in the ion reservoir 3 are simultaneously accelerated in the z direction and expelled as ion pulses. The ions are reflected by a mirror portion 9 mounted at an opposite position. Then, the ions travel toward a final ion detector 10 consisting of microchannel plates (MCPs) or the like.
Strictly, the ions have y-direction velocity components given when they are introduced into the ion reservoir 3. Therefore, if the ions undergo z-direction forces by the electric fields produced in the two-stage accelerating region 8, i.e., between the push-out plate 4 and the ion extraction grid 5 and between the grid 5 and the exit grid 6, the direction of travel is shifted to the y direction slightly from the z direction.
When the ions undergo the above-described acceleration, a given energy corresponding to the potential difference between the push-out plate 4 and the exit grid 6 is uniformly imparted to the ions and so ions of smaller masses have greater velocities and ions of greater masses have smaller velocities when the acceleration ends. Because of the velocity variations as described above, the ions are mass-dispersed while they are traveling through a reflectron TOF-MS spectrometer portion 12 placed at a negative potential V2 by a mass spectrometer portion power supply 11. Consequently, the ions are dispersed into ion pulses according to mass. As the ions having smaller mass-to-charge ratios (m/z; m: mass, z: valence number) reach the final ion detector 10 sooner, mass dispersion occurs. Thus, the ions can be observed as a mass spectrum.
A tandem MCP (a couple of MCPS), usually employed as an ion detector for TOF-MS to maintain an appropriate secondary electron multiple gain ranging 104-106, is made of millions of very thin capillary tubes of conductive glass bundled together, each having a diameter of 10 to 25 xcexcm and a length of 0.24 to 1.0 mm. Each tube acts as a secondary electron multiplier. Since secondary electrons travel only less than 1.0 mm in the microchannel plate, the plate can respond at a high speed of 1 nanosecond (ns) to applied pulsed, charged particles. On the other hand, where a photomultiplier tube or secondary electron multiplier tube where secondary electrons travel about several centimeters is used, a response time of about 5 ns is necessary.
Generally, the mass resolution of a time-of-flight mass spectrometer is given by:                     R        =                              M                          Δ              ⁢                              xe2x80x83                            ⁢              M                                =                                    t              TOF                                                      2                ·                Δ                            ⁢                              xe2x80x83                            ⁢              t                                                          (        1        )            
where M is mass in dalton (Da), xcex94M is a mass difference, tTOF is the flight time of ion M+, and xcex94t is the width of an ion pulse. The ion pulse width xcex94t is independent of the location on the Z-axis where a measurement is made. As the width in the final ion detector becomes narrowest, the mass resolution R is optimum. Accordingly, the width of the incident ion pulse is ideally equal to the width of the output signal from the secondary electron multiplier in the final ion detector. In practice, however, it is inevitable that the final ion detector itself will produce time spread, adding to the pulse width xcex94t in the denominator of Eq. (1).
Normally, in a high-resolution time-of-flight mass spectrometer, the pulse width xcex94t is about 5 ns at the entrance of the final ion detector. Since the time spread is roughly 5 ns as mentioned above where a photomultiplier or a secondary electron multiplier is used, the mass resolution R of the high-resolution TOF mass spectrometer is greatly affected. For example, when ions impinge on the final ion detector, consider the pulse width (t=5 ns) When leaving the final ion detector, the pulse width is temporally spread out to be (t=5+5=10 ns) Consequently, the mass resolution of the TOF mass spectrometer drops to xc2xd. For this reason, microchannel plates (MCPs) capable of responding in less than 1 ns are often used, especially in high-resolution TOF mass spectrometers.
In this case, however, the problem with the use of microchannel plates (MCPs) is that the linear range of output/input is limited in principle. In particular, the linearity of a microchannel plate is determined by a strip current value intrinsic in the microchannel plate. The linear range of output/input is narrower and indicated by three digits; in the case of a secondary electron multiplier, the range is wider and indicated by 5 digits. The strip current also acts to neutralize the electric charge of secondary electrons produced by the microchannel plate. It is known that the microchannel plate starts to saturate when the average output current of the microchannel plate is 5% to 6% of the strip current.
Of course, where the gain of the microchannel plate is set high, secondary electron-saturation of the microchannel plate tends to occur. Once such saturation takes place, the time taken to neutralize secondary electrons by the strip current is on the order of microseconds (xcexcs). If more secondary electrons are produced, the time is increased. The microchannel plate is insensitive, i.e., in a dead-time state, until the neutralization is completed. The outputs indicative of peaks of ions impinging during this insensitive period are zero. Peaks indicating these ions are absent from the mass spectrum. If the microchannel plate saturates repeatedly, deterioration of the microchannel plate is accelerated, thus shortening the lifetime.
As an example, it is assumed that a dead time of 1 xcexcs occurs. We now discuss what mass spectral range is absent from the produced ion spectrum. Where a monovalent ion having a mass of M Da (dalton) is accelerated by V volts and travels L cm through a free space, the flight time is approximated by:                               t          TOF                ≅                              0.72            ·            L                    ⁢                      xe2x80x83                    ⁢                                    (                              M                V                            )                                                          (        2        )            
where L is flight distance in cm, M is the mass of the ion in Da, and V is the accelerating voltage (in volts) for the ion.
Where a monovalent ion is accelerated with V=3000 volts, if flight distance L=100 cm, then the flight times tTOF of ions with masses M of 99 Da and 100 Da, respectively, are approximately 13.08 xcexcs and 13.14 xcexcs, respectively. The fight time difference per dalton is about 60 xcexcs. In this mass range, 1 xcexcs corresponds to a mass range of about 16.7 Da. Similarly, the flight times tTOF of ions having masses of 299 Da and 300 Da, respectively, are approximately 22.73 xcexcs and 22.77 xcexcs, respectively. The time difference per dalton is about 40 ns. In this mass range, 1 xcexcs corresponds to a mass range of about 25 Da. Therefore, a dead time on the order of microseconds due to saturation of the microchannel plate gives rise to absence of peaks in a considerably wide mass range from a mass spectrum.
In view of the foregoing circumstances, it is an object of the present invention to provide a time-of-flight (TOF) mass spectrometer which has microchannel plates (MCPs) and which can prevent difficulties that would normally be caused where intense ion pulses impinge on the microchannel plates to thereby saturate the microchannel plates, whereby the resulting dead time produces deficient mass spectra and shortens the lives of the microchannel plates.
This object is achieved in accordance with the teachings of the present invention by a time-of-flight (TOF) mass spectrometer comprising: an ion source for producing ion pulses; a time-of-flight mass spectrometer region through which the ion pulses emitted from the ion source travel; a final ion detector for detecting incident ion pulses which have traveled a given distance through said region and have been dispersed into plural ion pulses according to their own flight velocities; a flight time-measuring portion for measuring times taken for the dispersed ion pulses to reach the final ion source since departure from the ion source; an intermediate ion detector mounted in said time-of-flight mass spectrometer portion and acting to detect current values of said dispersed ion pulses before reaching the final ion detector; a measuring means for measuring elapsed times since the dispersed ion pulses reaching the intermediate ion detector have left the ion source; a computer means for forecasting the flight time at which the dispersed ion pulses will reach the final ion detector based on the measured elapsed times; and a saturation-preventing circuit for controlling the gain of the final ion detector according to the current values of the dispersed ion pulses detected by the intermediate ion detector and according to the forecast times of arrival of the dispersed ion pulses at the final ion detector in synchronism with arrival of the dispersed ion pulses to prevent the dispersed ion pulses from saturating the final ion detector.
Preferably, according to the present invention, the aforementioned intermediate ion detector is located at a spatial focusing point for ions in the time-of-flight mass spectrometer portion.
The aforementioned saturation-preventing circuit is characterized in that it switches the gain of the final ion detector between plural different values according to the current values of ion pulses.
Preferably, according to the present invention, there is further provided a storage means for storing the output signal from the final ion detector indicative of ion pulses, together with information about the gain during the detection, such that these two kinds of information stored can be correlated.
Preferably, according to the present invention, a mirror portion (reflectron) is mounted before the final ion detector and behind the intermediate ion detector.
The aforementioned ion source is characterized in that it is of the orthogonal acceleration type and comprises an external ion source for continuously emitting ions, an ion reservoir for introducing the ion beam emitted from the external ion source, and an ion accelerating region for accelerating the ion beam from the ion reservoir in a pulsed manner in a direction crossing the direction of introduction of the ion beam by application of a pulsed voltage.
The external ion source described above is any one of an electron impact (EI) ion source, a chemical ionization (CI) ion source, a fast atom bombardment (FAB) ion source, an electrospray ionization (ESI) ion source, an atmospheric pressure chemical ionization (APCI) source, and an inductively coupled plasma (ICP) ionization source.
The aforementioned final ion detector may be made of either microchannel plates (MCPs) or microsphere plates (MSPs).
The present invention also comprises a method of performing mass analysis with a TOF mass spectrometer comprising the use of an ion source for emitting ion pulses, a TOF mass spectrometer region through which the ion pulses emerging from the ion source travel, and a final ion detector on which ion pulses that have traveled through said region a given distance impinge. This method starts with measuring the current values of the ion pulses which are emitted from the ion source and traveling toward the final ion detector and the elapsed times since departure from the ion source. The gain of the final ion detector is controlled according to the measured elapsed times and the measured current values in synchronism with the arrival of the ion pulses. Thus, saturation of the final ion detector due to the ion pulses is prevented.
Preferably, according to the present invention, an intermediate ion detector is mounted at the spatial focusing point for the ions in the TOF mass spectrometer portion to measure both current values of the ion pulses and the elapsed times since departure of the ions from the ion source.
Preferably, according to the present invention, the gain of the final ion detector is switched between plural different values according to the current values of the ion pulses.
Preferably, according to the present invention, there is provided a storage means for storing the output signal from the final ion detector indicative of the ion pulses, together with information about the gain during the detection, such that these two kinds of information stored can be correlated.
Preferably, according to the present invention, there is provided a mirror portion located before the final ion detector and behind the intermediate ion detector.
In one embodiment of the present invention, the aforementioned ion source is an orthogonal acceleration ion, source comprising an external ion source for emitting ions continuously, an ion reservoir for introducing the ion beam emitted from the external ion source, and an ion accelerating region for accelerating the ion beam in a pulsed manner from the ion reservoir in a direction crossing the direction of introduction of the ion beam.
The aforementioned external ion source is any one of an electron impact (EI) ion source, a chemical ionization (CI) ion source, a fast atom bombardment (FAB) ion source, an electrospray ionization (ESI) ion source, an atmospheric pressure chemical ionization (APCI) source, and an inductively coupled plasma (ICP) ionization source.
The aforementioned final ion detector may be made of either microchannel plates (MCPs) or microsphere plates (MSPs).
Other objects and features of the invention will appear in the course of the description thereof, which follows.